Make sure you acknowledgement the day by just mentioning the special day that would have been. When losing one you have cherished & loved with all your heart those special days still have deep meaning & surely deserve to be honored by the surviving relatives and/or friends. My blessings are with your dad.
Spend the day with him. I don't know what he enjoys but if he has a favorite meal, make sure he gets it. If he has any special interests (art, model trains, gardening, classic cars, etc) try to take him somewhere to view those things such as a museum or formal garden. Also, spend some time with him looking at photo albums and try to get him to tell you some stories about your mom.
Take him to a park or their favorite spot, maybe her grave side and let him release some balloons in her honor and remembrance. He could maybe attach a card or hand written note, possibly a rose or her favorite flower. It would kind of be like still giving her a card and flowers on their special day, only he is sending them to her in heaven.
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